Toxoplasmosis is a rare but severe complication of bone marrow transplantation. Here, we report three patients in whom toxoplasmic pneumonitis developed, leading to fatal acute respiratory distress syndrome (ARDS). All patients had positive pretransplantation tests for Toxoplasma gondii and were therefore at risk to develop toxoplasmosis reactivation. They all recovered from aplasia, but soon after they died from brutal and severe ARDS. The possible role of an immunopathologic response to T gondii in the lungs in triggering ARDS is discussed.Early screening of parasitemia using highly sensitive polymerase chain reaction methods in seropositive patients with unexplained fever may be needed.